The firing mechanism of a firearm is generally constituted by a hammer which rotates about a pivot and is actuated by a spring.
One end of the hammer is provided with a hammer dog which is adapted to engage an associated trigger dog of the trigger, so that a rotation of the trigger leads to the disengagement of the hammer, which, being actuated by the spring, acts on the percussion pin.
In the firing action there are two critical moments in the action of the spring on the hamme: the first critical moment is the percussion step, during which the spring should preferably act with the maximum possible force on the hammer; the second critical moment is the hammer cocking step, which naturally must encounter the least possible resistance, especially in the case of automatic or semiautomatic firearms.
In known firing mechanisms, the spring is generally in the maximum compression position in the cocking step, whereas in the percussion step it is in its maximum extension position; in this manner it is apparent that the spring exerts excessive resistance during the cocking step, while not all the possible force is exerted on the hammer during the percussion step.
A few attempts to solve this problem have been made, at the cost of an excessively complex construction, however. It would be in fact easy to design a mechanism, with the desired characteristics, comprising several levers; such mechanism, however, would be too expensive and unreliable, as well as heavy and cumbersome.
A simpler design has been proposed in U.S. Pat. No. 4,693,027 which discloses a flat gun spring, for handguns. The spring has a progressively decreasing depth that permits more energy to be stored in the spring during the early stages of the hammer rotation and less energy in the later stages of the hammer rotation. This design substantially aims at eliminating "stack up" immediately prior to hammer release.